On WPCentral's lumia 630 review (which doesn't have such sensor), they state that it doesn't affect anything - apparently, MS found a way around, which means no face dial. I personally never tested it though

Cuz I'm gonna be porting to Cricket next week and the 630 seems to be their best free phone right now. Might as well get a free phone for the sim card to put in an unlocked Moto G. I'm actually gonna try the 630 for a while before purchasing the Moto G LTE.